The most common commercial form of injection molding machine is known as a “reciprocating screw”. In this type of machine, thermoplastic polymer is melted, mixed, and conveyed by means of a screw having one or more flights rotating within a heated pressure vessel. The screw is also permitted to translate axially to allow for the accumulation of melted material at the end of the screw. When sufficient melt has accumulated, the screw is stopped and translated forward to inject the melted material into a closed mold. In common practice a non-return valve is situated at the downstream end of the screw to prevent back flow into the screw flights during the injection portion of the cycle.
A variation of this process, known as co-injection or sandwich molding, has been commercially practiced for a number of years. As shown for example in FIG. 1, in its most common embodiment, co-injection is achieved by means of a molding machine fitted with two or more plasticizing units, each one containing a reciprocating screw enclosed in a separate heated barrel. The output of these plasticizing units is brought together by a system of manifolds which convey the several materials to the point of injection into the mold. According to well known principles of viscous flow, the first material to enter the mold remains substantially on the outside of the molded part, and material injected later remains substantially in the core of the part. The resulting “sandwich” construction yields a number of advantages, the principle advantages being: (1) to make a part with a chemically foamed core, gaining the light weight, low pressure, and flat surfaces of a foam part without the characteristic streaky exterior; (2) to use low cost recycled, “off-spec” or uncolored material where it is not visible; and (3) to make a part with different properties on the inside and outside, as for instance the presence or absence of reinforcing fibers or other property-changing additives.
These advantages are offset by the high cost and complexity of a machine requiring two or more independent reciprocating screws, together with the associated controls for simultaneous and/or sequential injection. As shown for example in FIG. 2, numerous attempts have therefore been made to reduce this complexity by having at least the injection function be performed by a single element, building a composite shot containing a plurality of melted materials within a single accumulation space. Examples include U.S. Pat. No. 4,978,493 to Kersemakers et al., U.S. Pat. No. 3,966,372 to Yasuike et al., and U.S. Pat. No. 5,443,378 to Jaroschek et al. In all of these examples of prior art, a secondary extrusion screw and barrel, or more than one, is caused to communicate with the primary barrel by means of some melt-carrying manifold structure through which the secondary portion of the shot is charged. Because of the multiple barrels and screw drives, machines of this type still have disadvantages involving the high initial cost of the required components and associated control capability.
The complexity of a multi-material machine is further reduced by this inventor's previous U.S. patent application Ser. No. 09/850,696, wherein two coaxial screw elements are used to plasticize two materials within the space of a single barrel. However, the cost and complexity of producing a screw with a sufficiently large central bore, and the associated issues of wear and steel strength, are potential limitations which could limit such a coaxial configuration to machines with large barrel diameters.